Family of Chiral Ferroelectric Compounds with Widely Tunable Band Gaps

Intense research activities have been exploring the possibility of harnessing solar energy via photovoltaic and photocatalytic applications of ferroelectric materials using the built-in electric field for an efficient separation of photoexcited charge carriers. However, one of the most important bottlenecks in these efforts is to find ferroelectricity in suitably low-band-gap materials for harvesting a sizable part of the solar spectrum, with most of the known ferroelectric materials having band gaps larger than 2.5 eV. In the present work, we show that the known chiral and polar compound, (R-/S-MBA)2CuCl4, with MBA = α-methyl benzylamine, is also ferroelectric and its ligand-to-metal charge transfer (LMCT) band gap (∼2.53 eV) can be systematically decreased via substitution of Cl- with Br- forming the solid solution (R-/S-MBA)2CuCl4-xBrx. These compounds retain their chiral ferroelectric state until x = 2 and reach a significantly low band gap of ∼2.09 eV for (R-/S-MBA)2CuCl2Br2, which is the smallest band gap reported so far among layered hybrid ferroelectric materials. We elucidate the origin of the band gap reduction and other changes in the electronic structure with the help of state-of-the-art electronic structure calculations. Chiral ferroelectrics constitute an interesting class of materials, with ferroelectricity being able to discriminate between electron and hole charge transports, while chirality may have the ability to discriminate between up- and down-spin transports.